A typical solid-state imaging device, such as a CCD image sensor, has an effective pixel area for receiving object light, and at least one optical black area (hereinafter, OB area) shielded from the object light. The OB area is provided for calculating a reference value (hereinafter, black reference level) to be used for correcting output signals from the effective pixel area. Calculation of this black reference level calculation and subsequent signal correcting process are generally performed in a so-called OB clamping circuit.
During a blanking period, the OB clamping circuit determines the black reference level by averaging the output signals (dark-current signals) generated from the OB area. Then, during a signal-output period of the effective pixel area, the OB clamping circuit subtracts the black reference level from the output signal of each light receiving element. This OB clamp processing is generally classified into two types: a line clamp processing performed upon the scanning of one horizontal line (row), and a frame clamp processing performed upon the scanning of one frame.
If the OB area contains one or more defective pixels (hereinafter, OB defective pixels), the black reference level becomes higher than it should be. As a result, the output signals from the effective pixel area are redundantly reduced in the OB clamp processing, and the resultant image is blackened (brightness of an image is lowered). In the line clamp processing, the OB defective pixel may have a significant impact since there are a few pixels on each horizontal line in the OB area. The horizontal line having the OB defective pixel is thus excessively blackened, and results in appearing as a black streak in the image.
Additionally, the CCD image sensors may have a deficiency called a line flaw which is a product of vertically extending defective pixels due to a deficiency of the vertical transfer path. In the event that such a line flaw occurs in the OB area, the brightness may change in all the horizontal lines crossing the line flaw.
To prevent the degradation of image due to the OB defective pixels, Japanese Patent Laid-open Publication No. 2004-350104 discloses an image processor which performs a median filter process before the OB clamp processing so as to correct the OB defective pixels. In addition, U.S. Pat. No. 7,098,950 B2 discloses a MOS type image sensor that stabilizes the output signals by connecting the reset terminals of all the light receiving elements on the same horizontal line across the effective pixel area and the OB area to a common potential-averaging line.
However, the image processor of the Publication No. 2004-350104 requires a separate filtering circuit for correcting the OB defective pixels, resulting in a cost rise. The image sensor of the U.S. Pat. No. 7,098,950 B2 needs to rearrange the pixel array to install the potential-averaging line. This special pixel array can only be achieved in the MOS type image sensors.